PVE::INotify: testable interface parser

read_etc_network_interfaces now opens all /proc files and
passes them as filehandles to the actual implementation
which is now is __read_etc_network_interfaces.

Similarly write_etc_network_interfaces now prints the
content string returned by the implementation now named
__write_etc_network_interfaces.

This way a testsuite can use the __* variants directly to
test the parser, passing its own versions of the /proc
files.